1. Field of the Invention
The present invention relates to a system for detecting stray light and more particularly to a filter eliminating undesired signal components (disturbance light and various optical and electrical noises and the like) included in a video signal obtained from a high sensitive photosensing device.
2. Description of the Related Arts
As a method of detecting undesired light (stray emission) generated in an electron gun of a cathode ray tube (CRT), there has generally been practiced a method to electrically extract the wanted signal by cascading stages of electronic circuits such as active filter circuits using semiconductor devices. The output of such an active filter becomes a signal which has much noise components as shown in FIG. 7 and is bad in linearity.
The method of detecting weak stray emission generated from the electron gun of a CRT using an active filter circuit and the like as described above has the following drawbacks:
1) The circuit becomes complicated and has many positions where adjustments are required and, hence, the filter characteristics are liable to vary with changes in temperature and the like.
2) It is difficult for the circuit to eliminate high-frequency induced noises.
3) Necessary signal is considerably attenuated by the circuit and hence the signal level is lowered.
4) Linearity of the signal is worsened by the circuit.